Optimisers, microinverters and string inverters, explained simply

The short answer

All three do the same core job. Your panels make direct current, your house runs on alternating current, and something has to convert one into the other. A string inverter does that once, centrally, for the whole array. Optimisers keep the central inverter and add a small box behind each panel to condition its output first. Microinverters skip the central box altogether and convert on the roof, panel by panel.

A string inverter: one box for the whole array

The panels are wired in series into a string, and the string runs down to a single inverter, usually in a loft or a garage. It is the simplest arrangement and the cheapest, which is why most UK homes have one.

The catch sits in the wiring. A string performs at the level of its weakest panel. If one module sits under a chimney shadow for two hours every afternoon, it does not only lose its own output for those two hours, it pulls the rest of the string down with it. On a roof with no shading that simply never happens, and the simplicity is pure gain.

Optimisers: a small box behind each panel

A power optimiser is a DC to DC unit fitted behind each module. It conditions that panel's output at the panel, then sends the power on to the central inverter. A shaded module drops on its own instead of dragging its neighbours down, and you get monitoring for each panel rather than one figure for the whole array.

Two approaches exist in the UK market. System optimisers such as the SolarEdge S series are matched to that manufacturer's own inverter, quote 99.5% weighted efficiency and carry a 25 year warranty, with the S440 rated for panels up to 490Wp and the S500 up to 550Wp. Add on optimisers such as the Tigo TS4-A-O work with any inverter brand, handle modules up to 700W, and can be fitted selectively on only the panels that are genuinely shaded rather than on all of them. The Tigo 25 year term depends on the system staying internet connected with Tigo monitoring, otherwise it is five years. Read the warranty condition, not just the headline number.

Microinverters: an inverter on every panel

A microinverter converts DC to AC on the back of the module, so every panel becomes its own independent generator. Shading or a fault on one panel has no effect on the rest. The Enphase IQ8 range, available in the UK since early 2025, puts out 240 to 380VA per unit, pairs with panels from 230W to 540W, quotes 97.5% efficiency and carries a 25 year warranty. There is no large central box to find space for, and no single component whose failure stops the whole array.

Specifications are manufacturer published figures for the products named above, checked at the date on this page.

When shading actually makes the difference

• A chimney, dormer or vent pipe that throws a moving shadow across part of the array through the day.
• Trees to the south or west, particularly ones that will be taller in ten years than they are now.
• A neighbouring roof, extension or flue that clips the panels in winter when the sun sits low.
• Panels split across two or more pitches facing different directions.
• A roof where access is awkward, so knowing which panel is underperforming saves a wasted visit later.

One thing has changed recently and it is worth knowing before optimisers are sold to you as standard. Some modern back contact panels now deal with partial shade inside the module, bypassing only the shaded cells rather than losing the panel. Longi's Hi-MO X10 is the clearest example. That does not remove the case for panel level electronics on a genuinely awkward roof, but it does weaken the blanket claim that any shade at all means an optimiser on every panel.

What this changes on your quote

The cost order is straightforward. A string inverter is the cheapest, optimisers sit in the middle, and microinverters are the dearest of the three. Ask for the difference in pounds in writing rather than accepting an upgrade as a given, and see it in proportion to the rest of the job in our cost guide.

Warranty is the other thing to look at. The leading optimisers and microinverters carry 25 year warranties, which matches the panels above them. String inverters typically carry shorter terms and are the component most likely to need replacing within the life of the array. That is not an argument against them, it is an argument for knowing the term on the specific model in your quote. Whichever route you take, the installation work itself is covered by the OVO guaranteed 10 year workmanship warranty.

1. Look at your roof at three times of day and note anything that casts a shadow on it.
2. If nothing does, and the panels all face one way, start from a plain string inverter.
3. If something does, ask whether optimisers on only the affected panels would do the job.
4. If the roof is split across several pitches, or access is difficult, put microinverters in the comparison.
5. Get the price difference between the options in writing before you decide.